1. Field of the Invention
The present invention is directed to a high-frequency receiver, particularly for a nuclear magnetic resonance apparatus, of the type having an analog input part with at least one analog mixer stage, the mixer stage being operable with an injection frequency, and having a following digitalization stage that includes an analog-to-digital converter for the digitalization of a signal from the analog input part with a sampling rate prescribed by a sampling frequency.
2. Description of the Prior Art
Diagnostics nuclear magnetic resonance apparatuses (magnetic resonance apparatus, MR apparatus) currently employ analog-to-digital converters in the receiver part in order to sample a high-frequency reception signal received from a reception antenna and supply it to an image computer. The dynamics, immunity to interference and bandwidth of the high-frequency receiver critically enter into the quality and flexibility obtainable with the nuclear magnetic resonance apparatus with respect to various applications.
Regardless of whether they are implemented as analog or digital receivers, such receivers are often realized as superheterodyne receivers wherein mixers transform the received nuclear magnetic resonance signal into an intermediate frequency range or into a base frequency range.
U.S. Pat. No. 5,170,123 discloses a high-frequency receiver of this type. The receiver is connected to a high-frequency antenna for the reception of nuclear magnetic resonance signals. The frequency of the reception signals lies on the order of magnitude of 20, 40 or 60 MHZ. After a pre-amplification, the reception signal is supplied to an analog mixer. The mixer converts the reception signal to a fixed intermediate frequency with 125 kHz. This means that the frequency of the local oscillator or the injection frequency for the mixer must differ from the center frequency of the reception signal by this constant intermediate frequency. After low-pass filtering, the reception signal converted to the intermediate frequency is supplied to an analog-to-digital converter that samples the signal with 500 kHz, i.e. a four-fold over-sampling with reference to the intermediate frequency. In particular, frequencies within a frequency band from 0 to 250 kHz can thus be digitalized. The injection frequencies supplied to the mixers and the sampling frequency of the analog-to-digital converter generate harmonics and harmonic combinations that, without anti-interference measures, are superimposed on the reception signal. The necessary anti-interference measures can be considerable.